docs: def format_error(self, field_name, message, index=None):
          """Override-able hook to format a single error message as an Error object.
  
          See: http://jsonapi.org/format/#error-objects
          """
          pointer = ['/data']
  
          if index is not None:
              pointer.append(str(index))
  
          relationship = isinstance(
              self.declared_fields.get(field_name), BaseRelationship,
          )
          if relationship:
              pointer.append('relationships')
          elif field_name != 'id':
              # JSONAPI identifier is a special field that exists above the attribute object.
              pointer.append('attributes')
  
          pointer.append(self.inflect(field_name))
  
          if relationship:
              pointer.append('data')
  
          return {
              'detail': message,
              'source': {
                  'pointer': '/'.join(pointer),
              },
          }